Most combustion methods are confronted by unwanted emissions of nitrogen oxides (NOx) in the combustion fumes. Nitrogen oxides have adverse effects on human beings and on the environment. For example, they are responsible for acid rain and play a significant role in the formation of atmospheric ozone.
European regulations are moving towards a substantial reduction in nitrogen oxide emissions. Because of this, manufacturers of combustion equipment, in particular burners, are constantly striving to limit nitrogen oxide emissions as much as possible. In 2011, according to the European LCP Directive (no. 2001-80-EC), the nitrogen oxide emission limit values (“ELV”) for large combustion plants were fixed at 200 mg/Nm3 at 3% O2 for existing industrial furnaces and 100 mg/Nm3 at 3% O2 for new furnaces. It is very likely that these ELVs will be reduced in the years to come.
There are many chemical pathways for the formation of nitrogen oxides. For natural gas combustion in furnaces, the two main contributors are thermal nitrogen oxides (Zeldovich mechanism) and prompt nitrogen oxides (Fenimore mechanism). The rate of thermal nitrogen oxide production is very dependent on the temperature. Formation increases significantly when the temperature in the reaction zone exceeds 1,500 K. As well as being dependent on the temperature, thermal nitrogen oxide formation is also dependent on the residence time in the hot zones.
Generally, given the relative contributions of the two types of nitrogen oxide formation, the initial focus is on reducing thermal nitrogen oxides, then the effect of the modifications on the prompt nitrogen oxides is observed.
Nitrogen oxide reductions can be achieved by means of two principles, referred to as “primary methods” and “secondary methods”. The primary methods consist of preventing the formation of nitrogen oxides, while the secondary methods are aimed at destroying the nitrogen oxides formed. The secondary methods have the disadvantages of high implementation costs for the reduction method, high levels of ammonia releases and decreased robustness for the plant.
Thus, the primary methods seem preferable. Most low nitrogen oxide emission furnace burners are based on non-premixing of the air and/or fuel (e.g. as described in U.S. Pat. No. 6,485,289). In this architecture, two reaction zones are created: a first rich zone, supplying the energy required to stabilize a second, lean, zone, diluted by the internal circulation of combustion products. A technology to further improve the performance of low nitrogen oxide emission burners consists of using an external recirculation of the combustion products. The combustion air is then pre-diluted with fumes (approximately 30% of the flow of fumes in the flue). This makes it possible to reduce the oxygen content in the oxidant flow and thus reduce the temperature peak and the thermal nitrogen oxides. U.S. Pat. No. 6,869,277 can be cited. However, these systems are complex, expensive and require frequent maintenance. For these reasons they are not installed very often.
Flameless combustion, also called “dilute combustion”, is a type of combustion making it possible to limit the temperature peaks, and thus to substantially reduce nitrogen oxide emissions. This combustion is based on an intense dilution of the oxidant and fuel jets by means of internal recirculations of products of combustion, generated by the jets of oxidant and fuel which are injected separately using high velocities. The dilution makes it possible to switch from an intense localized combustion to a more moderate combustion intensity. The high temperature of the products used as diluent allows the stability of this mode of combustion to be ensured. This type of combustion is characterized by a large-size heat emission zone, uniform temperature at the flame front, much lower temperature peaks and much smaller temperature fluctuations than in traditional combustion, reduced nitrogen oxide emissions and a much weaker link between acoustic waves and heat emission.
U.S. Pat. No. 5,154,599 describes an example of a flameless combustion burner. This document presents a regenerative burner architecture, i.e. the fumes are aspirated by the burner, their energy is stored in a heat reservoir and then transferred to the combustion air so as to heat it up to 1,200 K. Dilution of the reagents before combustion is ensured by having a considerable distance between the air and fuel injection points (at least twice the diameter of the central injector) and a high flow rate for fumes that recirculate (recirculation ratio greater than two for natural gas).
This technology is implemented industrially in methods known as “hot”, i.e. methods where the average temperature of the chamber is much higher than the self-ignition temperature of the fuel in question. Flameless oxidation is self-sustained by means of the self-ignition of an oxidant/fuel/burnt gasses ternary mixture. For self-ignition to occur, the temperature in the mixing zone must be higher than the self-ignition temperature. There are two possible ways of fulfilling this condition. In the first, at least one of the reagents (typically the oxidant) is preheated by means of energy recovered from the fumes or by means of an external energy source. In the second, internal recirculations of the hot combustion products are used to exceed the self-ignition temperature in the reagents mixing zone. These two stabilization methods are widespread in high-temperature applications (chamber temperature higher than the self-ignition temperature). In effect, in this case the fumes have sufficient energy to enable the fuel and/or the oxidant to be preheated to a high temperature and thus the self-ignition temperature to be exceeded in the mixing zone.
The stability of the dilute combustion, and therefore its sustainability, is jeopardized in “cool-wall” type methods. As the combustion products cool on contact with the walls, they do not let the self-ignition temperature be exceeded in the recirculating oxidant/fuel/combustion products mixing zone. Flameless oxidation, as utilized in high-temperature applications, cannot therefore be extended to cold-wall type chambers.
However, several technologies have been developed, but not yet deployed on an industrial scale, for furnace-type applications, in order to remove this barrier. One can cite stabilizations by means of:                a pilot flame,        a catalytic element for lowering the self-ignition temperature, or        preheating the combustion air to a high temperature.        
Each of these technologies has drawbacks, in terms of cost, performance, complexity and/or reliability.
The problem of flame stabilization in industrial environments is not specific to dilute combustion. For furnace types of chambers, the flame stabilization of “low nitrogen oxide” burners is often based on a primary intense rich combustion zone that helps to stabilize the lean combustion zone, whose characteristics are close (in terms of dilution of the air by combustion products) to those of a flameless combustion. U.S. Pat. No. 5,407,347 can be cited as a modern low nitrogen oxide burner technology. As a dilute combustion application, patent EP 1,850,067, which envisages stabilizing a highly dilute combustion by means of a pilot burner, can be cited. However, this type of stabilization has the inconvenience of creating hot zones that are potentially high nitrogen oxide emitters.
For internal combustion applications, “HCCi” (acronym for Homogeneous Charge Compression Ignition) combustion in gasoline engines, whose properties, in terms of mixing, are those of a dilute combustion, is performed by spark plugs. As an industrial burner operates continuously, spark ignition technology cannot be applied here.